The invention generally relates to a downhole measurement apparatus and technique.
Measurements typically are performed downhole on a periodic or continuous basis in a subterranean well for purposes of obtaining information about subterranean formations and the fluids present in these formations. These may include pressure, voltages/currents, gravity or force, gamma ray and nuclear magnetic resonance measurements, as just a few examples. Downhole measurements typically are performed before production begins for purposes of locating production zones.
To conduct downhole measurements in a cased well during production, sensors have been conventionally lowered via wireline electrically conductive cables and more recently positioned on the exterior wall of the well casing. For example, sensors that measure resistivity are traditionally positioned on the outside of an insulated well casing to measure the flow of currents through the surrounding formation(s). The casing-mounted sensors typically are mounted on the exterior of well casing sections before the well casing sections are installed downhole and are usually cemented in place. Each casing-mounted sensor is thus permanently installed, and thus, the sensor cannot be replaced if the sensor fails, a failure may become more likely over time. Other problems associated with sensors that are positioned on the exterior of the well casing include challenging issues relating to the placement of sensors and the routing of communication lines to the sensors. Problems associated with sensors lowered at the end of conductive cables include loss of production due to closing of well to make measurements, disruption of fluids one is trying to measure and inability to measure steady state flowing conditions due to need for modification of flow to lower cable etc, just to name a few.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.